1. Field of the Invention
This invention relates to complex filters, and more particularly to a complex filter topology that is based on a biquadratic topology but provides a tunable filter characteristic that is a function of transconductance and capacitance.
2. Description of the Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital amps, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter of a transceiver includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with the signal generated by one or more local oscillators to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver of a transceiver is also coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives an inbound RF signal via the antenna and amplifies it. The one or more intermediate frequency stages mix the amplified RF signal with the signal generated by one or more local oscillators to convert the amplified RF signal into a baseband signal or an intermediate frequency (IF) signal. This is typically referred to as frequency down-conversion. The filtering stage filters the down-converted baseband or IF signal to attenuate unwanted out of band signals to produce a filtered signal that is only that which falls within the bandwidth of the selected channel. Thus, this filter is sometimes referred to as a channel select filter. The data recovery stage recovers raw data from the filtered signal in accordance with the particular wireless communication standard.
As mentioned above, RF signals received by a wireless network transceiver are typically down-converted to intermediate or base-band frequencies by the transceiver before the carrier signals in the channel are processed and ultimately demodulated to extract the transmitted information. Down-conversion requires that the received RF signal be mixed with an oscillator signal having the desired intermediate or base-band frequency to which the received signal is to be down-converted.
The mixing process inherently generates a number of signal components in addition to the converted information signals. These include mirror image or sideband components, as well as distortion and intermodulation components, all of which can interfere with the processing of the received signals within the selected channel. Thus, it is important that the band-pass filter reject the interference components as well as those information signals transmitted in channels other than the selected channel. The degree of attenuation that must be achieved by the filter at frequencies outside the bandwidth of the selected channel is specified by the network designer.
Down-converting to a lower IF or baseband frequency renders implementation of a channel select band-pass filter that meets the performance characteristics specified for a particular wireless protocol significantly easier. For example, it is easier to implement a filter with a transfer function that meets the desired degree of roll-off (i.e. the rate of attenuation) at the frequency boundaries of a channel at IF or baseband frequencies than at RF frequencies. Moreover, the requisite sharpness of the roll-off can be more relaxed at the lower frequencies.
One of the difficulties presented by the integration of radio transceivers on monolithic integrated circuits is that it is difficult to steadily maintain the desired characteristic or transfer function of the channel select filter over the variations in circuit characteristics that result from variations in processing different batches of the integrated circuits, as well as those changes in circuit characteristics due to variations in the ambient temperature and supply voltage. Variations in these parameters can lead to the failure on the part of the channel select filter to maintain the required level of attenuation of the out-of-band signals to the levels specified for the particular design.
Another persistent goal of integrated circuit designers is to reduce the overall cost of system components such as a transceiver, which in turn lowers the cost of systems into which such components are integrated. As is well known, one of the most direct paths to reducing the cost of monolithic integrated circuit components is to minimize the die area consumed by the component's circuitry. Reduction in die area can be achieved in a number of ways, including the simple reduction in the number of components and therefore in the interconnect complexity of the integrated circuit.
Therefore, it would be desirable to provide a filter topology that can render an integrated circuit implementation of a high-order complex filter such as the band-pass channel select filter described above more simply and with less cost in terms of the die area it occupies. It would further be desirable if such a topology rendered the transfer function substantially constant in the face of variations in circuit characteristics typically encountered due to variations in temperature, supply voltage and processing parameters. Finally, it would be highly desirable if the topology could permit the independent tuning of the individual stages of the complex filter, thus rendering the implementation and control of the filter characteristic simple and accurate.